Bidirectional communication systems are ubiquitous in modern society, with one example being the telephone system, and another being bidirectional or interactive cable television. It is ordinarily advantageous to use a single transmission path for the bidirectional transmission, to reduce the number of conductors or optical fibers extending between the stations which are communicating. When a bidirectional signal path is used, the equipment at each end must ultimately separate the signals into incoming and outgoing portions, so that they may be applied to the appropriate one of a display device or earphone, and so that signals produced by a camera or microphone at the same location do not interfere with the signal received from a remote location. The separation of signals in response to their direction of transmission is accomplished by a device known as a hybrid. When operating properly in a telephone system, the hybrid isolates the signals flowing in different directions from each other, and prevents "echoes" or ringing which arise from a tendency toward signal oscillation which results from circulation of transmit signals in a receive unit which leak into the receive unit and back into the transmit path. Hybrids exhibit optimum performance, defined as transmit-to-receive isolation, when the minimum amount of transmit signal appears at the receive signal port. This maximum isolation is achieved when the bidirectional signal transmission path presents an impedance to the bidirectional signal port of the hybrid which matches the impedance for which the hybrid is designed. Desirable isolation may range from about 30 to 50 dB, depending upon the application. Isolations as low as 10 dB may result from transmission-path impedances which deviate by a factor of two or three from the design impedance. Thus, impedances which are two or three times the hybrid design impedance, or which are one-half or one-third the design impedance, may adversely affect the isolation.
Unfortunately, the impedance presented by the bidirectional signal path to the hybrid depends upon such factors as the number of terminal units, as for example telephone units, connected thereto, and the characteristics of the individual terminal units. The load impedance presented to a hybrid may change dynamically during operation, as might occur if an extension telephone is picked up during a conversation already in progress. The problems resulting from such impedance changes have in the past often been ignored, or corrected by digital echo cancellers. Digital echo cancellers rely on voice activation, and as a result may have short periods of echo which are detectable by the user.